Option to specify `rel="nofollow"` in links

[yandodov]

This was briefly mentioned in #92 here, but I decided to open a separate issue.

We need to put rel="nofollow" on all external links in our articles for better SEO. Currently, Kirby only adds rel="noopener noreferrer".

What's worse is that even if I add nofollow manually in the TXT file, it gets removed when I save the article later in the panel.

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Currently, I add nofollow with a custom paragraph.php snippet:

<p><?= str_replace('noopener noreferrer', 'noopener noreferrer nofollow', $content) ?></p>

...but it would be nice to be able to select whether you want nofollow or not.